The present invention relates to the fabrication of devices having a high degree of integration density, e.g. integrated circuits and/or micro-electromechanical devices (MEMs), among others, and more specifically to an improved system and method of chemically mechanically polishing a substrate on which such devices are fabricated.
Chemical mechanical polishing (CMP) has become an indispensable step in the fabrication of integrated circuits (ICs). In some steps of the fabrication process of ICs, later layers cannot be applied to a semiconductor substrate unless an earlier applied layer presents a planar surface. A CMP process is used to planarize such layers.
In some processes used in IC fabrication, it may be desired to planarize a surface of an oxide layer of a semiconductor substrate. In particular cases, the oxide layer fills spaces between portions of one or more underlying layers of a different material, and only the oxide filled spaces are to remain, as planarized to the underlying layer after polishing to remove the overlying oxide. For example, the oxide layer may fill trenches etched into a semiconductor substrate provided as isolating features, e.g. “shallow trench isolations,” and “isolation trenches” between electrically active areas of the substrate.
One of the challenges of planarizing such oxide layer to the level of an underlying layer is the efficient removal of step height differences of the oxide layer, and of the overlying oxide layer, i.e., the “overfill”, despite it varying in height across the surface of the substrate being polished. In the case of large step height and overfill, a reactive ion etching process (to reduce step height in the deposited dielectric material) is typically required as a prerequisite to conventional chemical-mechanical polishing with an abrasive slurry, to achieve satisfactory planarization. However, reactive ion etch processes are not desirable from the point of cost and/or process control. In addition, polishing with an abrasive slurry is prone to dishing, such that, in the process of removing features having step height differences and the overfill, some areas are overpolished to an extent that produces concave surfaces. Such is unsatisfactory in terms of achieving planarization of the surface being polished.
Some CMP methods which use fixed abrasive polishing pads rather than an abrasive slurry have been applied to the planarization of substrates having step height differences. However, where the step height difference is substantial, such fixed abrasive CMP methods are incapable of adequately removing the topography having step height differences and overfill without either overpolishing, or otherwise removing the overfill too slowly to be cost-effective. In such case, dishing of the oxide fill may result, or even scratching, ultimately producing an unsatisfactory surface. This deficiency has heretofore limited the use of such fixed abrasive CMP methods to structures having small, e.g., less than 200 Å variation in trench depth or oxide overfill across the substrate. In some fixed abrasive CMP methods, the material removal rate following the onset of planarization, i.e. after removing the features having step height differences, has often been unsatisfactorily low.
Thus, there is a need for an improved chemical mechanical polishing process capable of more rapidly planarizing an oxide material having step height differences and/or differences in overfill levels to produce a substantially planar surface having few scratches, while avoiding the need for RIE processing or other undesirable alternatives.